Rev Environ Health 2014; 29(4): 363–378
Review Shedrack R. Nayebare*, Lloyd R. Wilson, David O. Carpenter, David M. Dziewulski and Kurunthachalam Kannan
A review of potable water accessibility and sustainability issues in developing countries – case study of Uganda Abstract: Providing sources of sustainable and quality potable water in Uganda is a significant public health issue. This project aimed at identifying and prioritizing possible actions on how sustainable high quality potable water in Uganda’s water supply systems could be achieved. In that respect, a review of both the current water supply systems and government programs on drinking water in Uganda was completed. Aspects of quantity, quality, treatment methods, infrastructure, storage and distribution of water for different water systems were evaluated and compared with the existing water supply systems in the U.S., Latin America and the Caribbean, for purposes of generating feasible recommendations and opportunities for improvement. Uganda utilizes surface water, groundwater, and rainwater sources for consumption. Surface water covers 15.4% of the land area and serves both urban and rural populations. Lake Victoria contributes about 85% of the total fresh surface water. Potable water quality is negatively affected by the following factors: disposal of sewage and industrial effluents, agricultural pesticides and fertilizers, and surface run-offs during heavy rains. The total renewable groundwater resources in Uganda are estimated to be 29 million m3/year with about 20,000 boreholes, 3000 shallow-wells and 200,000 springs, serving more than 80% of the rural and slum communities. Mean annual rainfall *Corresponding author: Shedrack R. Nayebare, Department of Environmental Health Sciences, School of Public Health, University at Albany, SUNY, Albany, NY 12201, USA, E-mail: [email protected]; [email protected] Lloyd R. Wilson and David M. Dziewulski: Department of Environmental Health Sciences, School of Public Health, University at Albany, SUNY, Albany, NY, USA; and Bureau of Water Supply and Protection, New York State Department of Health (NYSDOH), Albany, NY, USA David O. Carpenter: Institute for the Health and the Environment, University at Albany, Rensselaer, NY, USA Kurunthachalam Kannan: Department of Environmental Health Sciences, School of Public Health, University at Albany, SUNY, Albany, NY, USA; and Wadsworth Center, New York State Department of Health, Albany, NY, USA
in Uganda ranges from 500 mm to 2500 mm. Groundwater and rainwater quality is mainly affected by poor sanitation and unhygienic practices. There are significant regional variations in the accessibility of potable water, with the Northeastern region having the least amount of potable water from all sources. Uganda still lags behind in potable water resource development. Priorities should be placed mainly on measures available for improvement of groundwater and rainwater resource utilization, protection of watersheds, health education, improved water treatment methods and distribution in rural areas, and pollution control and monitoring. Implementing these changes can promote potable water accessibility especially to the poor populations living in rural and urban slum areas because they comprise the majority (80%) of Uganda’s population. Keywords: government legislations; potable water; water disinfection; water distribution; water pollution; water systems (surface water, groundwater and rainwater). DOI 10.1515/reveh-2013-0019 Received December 3, 2013; accepted April 23, 2014; previously published online June 11, 2014
Background and introduction Despite numerous surface and groundwater sources in the country, Uganda’s population still faces intermittent shortages in potable water supply, and most of those that have a continuous supply are served water originating from eutrophic surface water sources. To date, the goals related to water supply have been to improve the accessibility and continuity of the supply and to deal with the acute health concerns pertaining to water quality issues (i.e., microbial contamination, color, turbidity, and chemical contaminants). As the water supply develops and improves to mitigate acute issues like biological contamination, more
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364 Nayebare et al.: Potable water issues in developing countries emphasis may be placed on the chronic health concerns associated with contaminants like the disinfection byproducts (DBPs). In this review, we aimed at identifying the gaps in Uganda’s water supplies and suggest some feasible recommendations for improvement. We evaluated different water supply systems (i.e., surface water, groundwater, and rainwater) as well as the drinking water legislation, so as to clearly define the current state of Uganda’s water supply. Our review covers the aspects of quantity, quality, treatment methods, infrastructure, storage, and distribution of water for the different water systems in Uganda. This review also attempts to compare these with water supply systems and regulations in the U.S., Latin America, and the Caribbean countries. Details of our reviews on water supply systems and legislation for the U.S., Latin America and the Caribbean countries can be found online at http://gradworks.umi.com/15/12/1512996.html. On the one hand, the U.S. was considered for comparison due to the extensive current research in water development conducted in the country and the efficient water supply programs and laws/legislation that could be used to suggest improvements in Uganda. On the other hand, Latin American and Caribbean countries have relatively similar settings as Uganda in terms of potable water accessibility, as defined by several criteria like water resource distribution, water pollution issues, and public awareness challenges related to sanitation and hygiene (1). Thus, it would be suitable to recommend practices appropriate for Uganda, which have been effectively implemented in these countries. Furthermore, in an effort to devise more feasible recommendations on future water development plans that are applicable to Uganda and other developing countries, an analysis of the cost-benefit and obstacles present to implementation of the activities aimed at improving water supplies in Uganda was completed.
Surface water Accessibility issues Despite the existence of a vast number of lakes (about 27 in total), rivers and streams (Figure 1), Uganda continues to face the issue of limited accessibility to potable water, particularly in rural and poor urban communities. Uganda’s surface water sources cover 15.4% of the total land area (236,040 km2) and these provide domestic water supply to both urban and rural populations (3). The current population of Uganda stands at 34.76 million people, of which about 20% reside in urban areas (4). It is estimated that about 71% of the urban population have access to potable
Figure 1 Map of Uganda Showing Major Lakes, Rivers and Regions of the Country (2).
water distributed by the National Water and Sewerage Corporation (NWSC) (5), which essentially obtains its raw water from lakes. Urban supply of domestic water is delivered through piped household water connections and communal tap-stands using an approved fee schedule of the Ministry of Water and Environment (MWE). Those living in rural and suburban communities comprise about 80% of Uganda’s population. The NWSC does not extend to these communities, so the surface water is mainly obtained from rivers, streams, swamps, seepages, ponds, and sometimes floods during rainy seasons. Some of the surface water in rural areas comes from unprotected springs that form stagnant ponds. With the exception of communities living on lake shores, many surface water sources in rural areas are seasonal, and thus, are mostly available and utilized during the rainy seasons. Quality issues Water pollution ensuing mostly from anthropogenic activities (e.g., agriculture, industrialization, and sewage discharge) remains a significant impediment to potable water supply in Uganda. In fact, most fresh surface water sources
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Nayebare et al.: Potable water issues in developing countries 365
rate poorly in terms of bacteriological and chemical quality. In rural and slum communities, the major issues with potable water are mostly related to poor bacteriological quality. Such poor quality is due to unhygienic practices characterized by widespread use of poorly constructed and sited pit latrines/toilets and dilapidated sewage systems, as well as the complete lack of sanitary facilities where open defecation in the fields is practiced in Uganda (6) and other neighboring countries (7). The majority of the people in these communities are living under the poverty line (earning just about 1US$ per day), which significantly impedes the execution of activities aimed at improving potable water supplies due to sustainability issues.
Case study on Lake Victoria With a surface area of 68,800 km2 (mean and maximum depths of 40 m and 79 m, respectively) and a total catchment area of 195,000 km2 (8), Lake Victoria (L. Victoria) is the second largest freshwater lake in the world and contributes about 85% of the total fresh surface water in Uganda. The lake is a shared resource among the countries of Uganda, Tanzania, and Kenya in proportions of 45%, 49% and 6% of the surface area, respectively (3, 8). It is a vital source of water supply for both domestic and industrial purposes in the East African region and supports the livelihood of millions of people (8). The NWSC draws its raw water from this lake and uses it to serve more than 10 million people living in over 25 districts within the lake basin area (4). This lake is eutrophic due to excessive inflows of nutrients (mainly phosphorous and nitrogen), which has promoted the growth of the common water hyacinth (Eichhornia crassipes) and algae (9). Since the 1960s, there has been a five-fold increase in algal growth on the
lake (10, 11) and as a result, the water appears green with high levels of total organic carbon (TOC) and poor microbial quality (12). Water pollution on the lake is essentially due to the discharge of raw and partially treated sewage and dumping of domestic and industrial chemicals as well as organic waste and fertilizers from several agricultural farms in the lake basin. There are several medium-to-large scale industries and numerous small-scale enterprises located on the L. Victoria catchment area; these discharge high organic, nutrient-rich effluents loaded with metals and organic pollutants into the lake (13) without any prior onsite pre-treatment. In addition, the Lake contains inhabited islands (3000 Ssese Islands), and native fishing populations dump their excreta directly into the water. Kampala City, the capital of Uganda, is located within the catchment area of L. Victoria. Most of the wetlands around the lake have been cleared for farming and human settlement, while the industrial discharges and storm water containing urban run-off from this city flows untreated into the Nakivubo channel, which drains directly into L. Victoria’s Inner Murchison Bay (14). At the Nakivubo channel discharge point into the lake, biochemical oxygen demand (BOD) is > 3.5 milligrams per liter (mg L–1), which indicates significant organic loading. Meanwhile, phosphorus levels range from 1.0 to 4.0 mg L–1 (15), representing values that are significantly greater than the World Health Organization (WHO) recommended maximum tolerance level of 0.1 mg L–1 for surface water. This has been closely linked to the occurrence of widespread and severe algal blooms within the bay (15), intensive color, turbidity levels of up to 84.0 nephelometric turbidity units (NTUs), and pathogenic microorganisms occurring in the Lake (16). General water quality parameters in Lake Victoria are summarized in Table 1.
Table 1 Summary of some of the water quality parameters in L. Victoria. Parameter
Recorded levels
Turbidity Biological oxygen demand (BOD5) Chemical oxygen demand (COD) Phosphorous (P) pH Nitrogen (N)
Up to 84.0 NTU 61–475 mg L–1 506–3877 mg L–1 2.4 mg L–1 5.0 mg L–1 4.0 mg L–1 0.94–7.0 mg L–1 7.0–9.9 4.8–10.8 mg L–1 as NO3– 2.6–10.8 mg L–1 as NH4+ 0.02–0.05 mg L–1 0.02–0.91 mg L–1
Lead (Pb) Manganese (Mn)
References Balirwa et al. (16) Oguttu et al. (17) Oguttu et al. (17) Awange and Obiero (18) Oguttu et al. (17) Oyoo (15) Madadi et al. (19) Oguttu et al. (17) Oguttu et al. (17) Oguttu et al. (17) Oguttu et al. (17)
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366 Nayebare et al.: Potable water issues in developing countries
Surface water treatment As outlined below, modern surface water treatment methods are employed by the NWSC in the treatment of water distributed in urban areas. The NWSC has three water treatment plants (Gaba I, II, and III) supplying treated water to Kampala City and the surrounding areas. In addition, the NWSC operates at least one water treatment plant for every major town across the country, from which the residents are supplied treated water. In rural areas and slum communities, water treatment, if any, is mainly done by boiling at household level, and to a small extent, by performing household-based water filtration and use of chlorine tablets and solar disinfection (SODIS), as discussed in the groundwater section. Filtration The National Water and Sewerage Corporation (NWSC) usually performs water filtration in three stages – the first two stages screen using trash racks and inlet screens, and the third stage is rapid gravity filtration using fine sand. The complete filtration process involves screening, coagulation, flocculation, sedimentation, and rapid gravity filtration. Screening A set of mechanical screens (trash racks, 3 inches wide) and a second set with smaller openings are installed at the inlet point into the drinking water treatment plant. This is meant to screen out large organic and un-decomposed materials like wood, plastic, and dead fish. This reduces water turbidity and ultimately prevents coarse materials from entering the remainder of the treatment process.
Coagulation This step involves the destabilization of suspended particles by surface charge neutralization to promote agglomeration into larger particles for easy removal (20, 21). Water is dosed with alum [Al2(SO4)3] at a relatively low concentration of ∼5 mg per liter (mg L–1) (22, 23) to facilitate agglomeration (24), while constantly monitoring pH for efficient solubilization of aluminum species. Optimum sweep coagulation occurs when negatively-charged forms of alum predominate, which occurs at pH 6–8 (20, 21).
Flocculation The agglomeration of small suspended particles into larger particles for easy removal follows the coagulation step
(20). The water flows into a flocculation chamber where it is retained for 10–30 min to allow complete agglomeration into larger particles that can easily settle out of the water (20, 24). Several factors determine the efficiency of coagulation and flocculation processes, including type and dosage of coagulant, pH, flocculator retention time, water flow, intensity, and duration of mixing.
Sedimentation Efficient flocculation leads to sedimentation where suspended particles settle out of the water and come to rest at the bottom. The NWSC uses gravitational settling during water treatment. Here, the water passes through a clarifier with little disturbance, where the macro-flocs settle to multiple parallel plates (lamellar settler) or to the bottom of a vessel and clear water flows out over an effluent baffle into the rapid-gravity sand filtration bed. The solids collected at the bottom are then removed by a mechanical “sludge collection” device and pumped into sludge drying beds. Clarifiers are usually shallow at the inlets but become deeper towards the outlets so as to ensure quiescent conditions that promote settling.
Rapid gravity filtration The water exiting the clarifier is allowed to trickle through a fine sand filter bed. This helps to further strain out any suspended particles and some microorganisms that would have survived the initial processes before the water is chlorinated. The filter bed is cleaned/ maintained by backwashing and periodic scrapping off of the clogged upper layer of sand and replacing it with clean sand. The frequency of cleaning depends mainly on the frequency of use, but is usually done every 72 h of constant operation.
Disinfection Public water supplies primarily use chlorination. The disinfection methods applied for surface water in rural Uganda differ from those used in urban areas. The urban populations mainly utilize water supplied by the NWSC, which is chlorinated prior to distribution. Water is treated so that a free chlorine residual of 0.5 mg L–1 is maintained throughout the distribution. However, for the most part this is never attained in practice. Treated water is monitored for: pH 6.7–7.2; true-color less than (
Review Shedrack R. Nayebare*, Lloyd R. Wilson, David O. Carpenter, David M. Dziewulski and Kurunthachalam Kannan
A review of potable water accessibility and sustainability issues in developing countries – case study of Uganda Abstract: Providing sources of sustainable and quality potable water in Uganda is a significant public health issue. This project aimed at identifying and prioritizing possible actions on how sustainable high quality potable water in Uganda’s water supply systems could be achieved. In that respect, a review of both the current water supply systems and government programs on drinking water in Uganda was completed. Aspects of quantity, quality, treatment methods, infrastructure, storage and distribution of water for different water systems were evaluated and compared with the existing water supply systems in the U.S., Latin America and the Caribbean, for purposes of generating feasible recommendations and opportunities for improvement. Uganda utilizes surface water, groundwater, and rainwater sources for consumption. Surface water covers 15.4% of the land area and serves both urban and rural populations. Lake Victoria contributes about 85% of the total fresh surface water. Potable water quality is negatively affected by the following factors: disposal of sewage and industrial effluents, agricultural pesticides and fertilizers, and surface run-offs during heavy rains. The total renewable groundwater resources in Uganda are estimated to be 29 million m3/year with about 20,000 boreholes, 3000 shallow-wells and 200,000 springs, serving more than 80% of the rural and slum communities. Mean annual rainfall *Corresponding author: Shedrack R. Nayebare, Department of Environmental Health Sciences, School of Public Health, University at Albany, SUNY, Albany, NY 12201, USA, E-mail: [email protected]; [email protected] Lloyd R. Wilson and David M. Dziewulski: Department of Environmental Health Sciences, School of Public Health, University at Albany, SUNY, Albany, NY, USA; and Bureau of Water Supply and Protection, New York State Department of Health (NYSDOH), Albany, NY, USA David O. Carpenter: Institute for the Health and the Environment, University at Albany, Rensselaer, NY, USA Kurunthachalam Kannan: Department of Environmental Health Sciences, School of Public Health, University at Albany, SUNY, Albany, NY, USA; and Wadsworth Center, New York State Department of Health, Albany, NY, USA
in Uganda ranges from 500 mm to 2500 mm. Groundwater and rainwater quality is mainly affected by poor sanitation and unhygienic practices. There are significant regional variations in the accessibility of potable water, with the Northeastern region having the least amount of potable water from all sources. Uganda still lags behind in potable water resource development. Priorities should be placed mainly on measures available for improvement of groundwater and rainwater resource utilization, protection of watersheds, health education, improved water treatment methods and distribution in rural areas, and pollution control and monitoring. Implementing these changes can promote potable water accessibility especially to the poor populations living in rural and urban slum areas because they comprise the majority (80%) of Uganda’s population. Keywords: government legislations; potable water; water disinfection; water distribution; water pollution; water systems (surface water, groundwater and rainwater). DOI 10.1515/reveh-2013-0019 Received December 3, 2013; accepted April 23, 2014; previously published online June 11, 2014
Background and introduction Despite numerous surface and groundwater sources in the country, Uganda’s population still faces intermittent shortages in potable water supply, and most of those that have a continuous supply are served water originating from eutrophic surface water sources. To date, the goals related to water supply have been to improve the accessibility and continuity of the supply and to deal with the acute health concerns pertaining to water quality issues (i.e., microbial contamination, color, turbidity, and chemical contaminants). As the water supply develops and improves to mitigate acute issues like biological contamination, more
Brought to you by | HEC Bibliotheque Maryriam ET J. Authenticated Download Date | 6/9/15 11:22 AM
364 Nayebare et al.: Potable water issues in developing countries emphasis may be placed on the chronic health concerns associated with contaminants like the disinfection byproducts (DBPs). In this review, we aimed at identifying the gaps in Uganda’s water supplies and suggest some feasible recommendations for improvement. We evaluated different water supply systems (i.e., surface water, groundwater, and rainwater) as well as the drinking water legislation, so as to clearly define the current state of Uganda’s water supply. Our review covers the aspects of quantity, quality, treatment methods, infrastructure, storage, and distribution of water for the different water systems in Uganda. This review also attempts to compare these with water supply systems and regulations in the U.S., Latin America, and the Caribbean countries. Details of our reviews on water supply systems and legislation for the U.S., Latin America and the Caribbean countries can be found online at http://gradworks.umi.com/15/12/1512996.html. On the one hand, the U.S. was considered for comparison due to the extensive current research in water development conducted in the country and the efficient water supply programs and laws/legislation that could be used to suggest improvements in Uganda. On the other hand, Latin American and Caribbean countries have relatively similar settings as Uganda in terms of potable water accessibility, as defined by several criteria like water resource distribution, water pollution issues, and public awareness challenges related to sanitation and hygiene (1). Thus, it would be suitable to recommend practices appropriate for Uganda, which have been effectively implemented in these countries. Furthermore, in an effort to devise more feasible recommendations on future water development plans that are applicable to Uganda and other developing countries, an analysis of the cost-benefit and obstacles present to implementation of the activities aimed at improving water supplies in Uganda was completed.
Surface water Accessibility issues Despite the existence of a vast number of lakes (about 27 in total), rivers and streams (Figure 1), Uganda continues to face the issue of limited accessibility to potable water, particularly in rural and poor urban communities. Uganda’s surface water sources cover 15.4% of the total land area (236,040 km2) and these provide domestic water supply to both urban and rural populations (3). The current population of Uganda stands at 34.76 million people, of which about 20% reside in urban areas (4). It is estimated that about 71% of the urban population have access to potable
Figure 1 Map of Uganda Showing Major Lakes, Rivers and Regions of the Country (2).
water distributed by the National Water and Sewerage Corporation (NWSC) (5), which essentially obtains its raw water from lakes. Urban supply of domestic water is delivered through piped household water connections and communal tap-stands using an approved fee schedule of the Ministry of Water and Environment (MWE). Those living in rural and suburban communities comprise about 80% of Uganda’s population. The NWSC does not extend to these communities, so the surface water is mainly obtained from rivers, streams, swamps, seepages, ponds, and sometimes floods during rainy seasons. Some of the surface water in rural areas comes from unprotected springs that form stagnant ponds. With the exception of communities living on lake shores, many surface water sources in rural areas are seasonal, and thus, are mostly available and utilized during the rainy seasons. Quality issues Water pollution ensuing mostly from anthropogenic activities (e.g., agriculture, industrialization, and sewage discharge) remains a significant impediment to potable water supply in Uganda. In fact, most fresh surface water sources
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Nayebare et al.: Potable water issues in developing countries 365
rate poorly in terms of bacteriological and chemical quality. In rural and slum communities, the major issues with potable water are mostly related to poor bacteriological quality. Such poor quality is due to unhygienic practices characterized by widespread use of poorly constructed and sited pit latrines/toilets and dilapidated sewage systems, as well as the complete lack of sanitary facilities where open defecation in the fields is practiced in Uganda (6) and other neighboring countries (7). The majority of the people in these communities are living under the poverty line (earning just about 1US$ per day), which significantly impedes the execution of activities aimed at improving potable water supplies due to sustainability issues.
Case study on Lake Victoria With a surface area of 68,800 km2 (mean and maximum depths of 40 m and 79 m, respectively) and a total catchment area of 195,000 km2 (8), Lake Victoria (L. Victoria) is the second largest freshwater lake in the world and contributes about 85% of the total fresh surface water in Uganda. The lake is a shared resource among the countries of Uganda, Tanzania, and Kenya in proportions of 45%, 49% and 6% of the surface area, respectively (3, 8). It is a vital source of water supply for both domestic and industrial purposes in the East African region and supports the livelihood of millions of people (8). The NWSC draws its raw water from this lake and uses it to serve more than 10 million people living in over 25 districts within the lake basin area (4). This lake is eutrophic due to excessive inflows of nutrients (mainly phosphorous and nitrogen), which has promoted the growth of the common water hyacinth (Eichhornia crassipes) and algae (9). Since the 1960s, there has been a five-fold increase in algal growth on the
lake (10, 11) and as a result, the water appears green with high levels of total organic carbon (TOC) and poor microbial quality (12). Water pollution on the lake is essentially due to the discharge of raw and partially treated sewage and dumping of domestic and industrial chemicals as well as organic waste and fertilizers from several agricultural farms in the lake basin. There are several medium-to-large scale industries and numerous small-scale enterprises located on the L. Victoria catchment area; these discharge high organic, nutrient-rich effluents loaded with metals and organic pollutants into the lake (13) without any prior onsite pre-treatment. In addition, the Lake contains inhabited islands (3000 Ssese Islands), and native fishing populations dump their excreta directly into the water. Kampala City, the capital of Uganda, is located within the catchment area of L. Victoria. Most of the wetlands around the lake have been cleared for farming and human settlement, while the industrial discharges and storm water containing urban run-off from this city flows untreated into the Nakivubo channel, which drains directly into L. Victoria’s Inner Murchison Bay (14). At the Nakivubo channel discharge point into the lake, biochemical oxygen demand (BOD) is > 3.5 milligrams per liter (mg L–1), which indicates significant organic loading. Meanwhile, phosphorus levels range from 1.0 to 4.0 mg L–1 (15), representing values that are significantly greater than the World Health Organization (WHO) recommended maximum tolerance level of 0.1 mg L–1 for surface water. This has been closely linked to the occurrence of widespread and severe algal blooms within the bay (15), intensive color, turbidity levels of up to 84.0 nephelometric turbidity units (NTUs), and pathogenic microorganisms occurring in the Lake (16). General water quality parameters in Lake Victoria are summarized in Table 1.
Table 1 Summary of some of the water quality parameters in L. Victoria. Parameter
Recorded levels
Turbidity Biological oxygen demand (BOD5) Chemical oxygen demand (COD) Phosphorous (P) pH Nitrogen (N)
Up to 84.0 NTU 61–475 mg L–1 506–3877 mg L–1 2.4 mg L–1 5.0 mg L–1 4.0 mg L–1 0.94–7.0 mg L–1 7.0–9.9 4.8–10.8 mg L–1 as NO3– 2.6–10.8 mg L–1 as NH4+ 0.02–0.05 mg L–1 0.02–0.91 mg L–1
Lead (Pb) Manganese (Mn)
References Balirwa et al. (16) Oguttu et al. (17) Oguttu et al. (17) Awange and Obiero (18) Oguttu et al. (17) Oyoo (15) Madadi et al. (19) Oguttu et al. (17) Oguttu et al. (17) Oguttu et al. (17) Oguttu et al. (17)
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366 Nayebare et al.: Potable water issues in developing countries
Surface water treatment As outlined below, modern surface water treatment methods are employed by the NWSC in the treatment of water distributed in urban areas. The NWSC has three water treatment plants (Gaba I, II, and III) supplying treated water to Kampala City and the surrounding areas. In addition, the NWSC operates at least one water treatment plant for every major town across the country, from which the residents are supplied treated water. In rural areas and slum communities, water treatment, if any, is mainly done by boiling at household level, and to a small extent, by performing household-based water filtration and use of chlorine tablets and solar disinfection (SODIS), as discussed in the groundwater section. Filtration The National Water and Sewerage Corporation (NWSC) usually performs water filtration in three stages – the first two stages screen using trash racks and inlet screens, and the third stage is rapid gravity filtration using fine sand. The complete filtration process involves screening, coagulation, flocculation, sedimentation, and rapid gravity filtration. Screening A set of mechanical screens (trash racks, 3 inches wide) and a second set with smaller openings are installed at the inlet point into the drinking water treatment plant. This is meant to screen out large organic and un-decomposed materials like wood, plastic, and dead fish. This reduces water turbidity and ultimately prevents coarse materials from entering the remainder of the treatment process.
Coagulation This step involves the destabilization of suspended particles by surface charge neutralization to promote agglomeration into larger particles for easy removal (20, 21). Water is dosed with alum [Al2(SO4)3] at a relatively low concentration of ∼5 mg per liter (mg L–1) (22, 23) to facilitate agglomeration (24), while constantly monitoring pH for efficient solubilization of aluminum species. Optimum sweep coagulation occurs when negatively-charged forms of alum predominate, which occurs at pH 6–8 (20, 21).
Flocculation The agglomeration of small suspended particles into larger particles for easy removal follows the coagulation step
(20). The water flows into a flocculation chamber where it is retained for 10–30 min to allow complete agglomeration into larger particles that can easily settle out of the water (20, 24). Several factors determine the efficiency of coagulation and flocculation processes, including type and dosage of coagulant, pH, flocculator retention time, water flow, intensity, and duration of mixing.
Sedimentation Efficient flocculation leads to sedimentation where suspended particles settle out of the water and come to rest at the bottom. The NWSC uses gravitational settling during water treatment. Here, the water passes through a clarifier with little disturbance, where the macro-flocs settle to multiple parallel plates (lamellar settler) or to the bottom of a vessel and clear water flows out over an effluent baffle into the rapid-gravity sand filtration bed. The solids collected at the bottom are then removed by a mechanical “sludge collection” device and pumped into sludge drying beds. Clarifiers are usually shallow at the inlets but become deeper towards the outlets so as to ensure quiescent conditions that promote settling.
Rapid gravity filtration The water exiting the clarifier is allowed to trickle through a fine sand filter bed. This helps to further strain out any suspended particles and some microorganisms that would have survived the initial processes before the water is chlorinated. The filter bed is cleaned/ maintained by backwashing and periodic scrapping off of the clogged upper layer of sand and replacing it with clean sand. The frequency of cleaning depends mainly on the frequency of use, but is usually done every 72 h of constant operation.
Disinfection Public water supplies primarily use chlorination. The disinfection methods applied for surface water in rural Uganda differ from those used in urban areas. The urban populations mainly utilize water supplied by the NWSC, which is chlorinated prior to distribution. Water is treated so that a free chlorine residual of 0.5 mg L–1 is maintained throughout the distribution. However, for the most part this is never attained in practice. Treated water is monitored for: pH 6.7–7.2; true-color less than (
